The standard spirit level which is typically used to determine if a surface is level or plumb provides a rugged and accurate means of accomplishing its purpose. In situations where an angle other than level or plumb is to be measured the spirit level does not provide the necessary flexibility, and in many cases it compromises accuracy. In recent years electronic inclinometers have been developed which provide a digital readout of angular displacement. Some of these inclinometers measure the relative angle using analog techniques, with the analog signal being converted to a digital signal using well-known methods. An example would be the AngleStar.TM. manufactured by Sperry, which uses a dielectric fluid between capacitance plates to sense angular position. The position of the liquid between the plates is a function of the angle of the surface being measured and thus modifies the inter-plate dielectric and, hence, the capacitance. After the analog signal is converted to a digital signal the electronic circuitry provides a digital readout of the surface angle over a limited angular rotation. Another digital inclinometer is presently marketed by Heath (Zenith), Model SM-2370, which uses a pendulum system to which is attached an encoder disk such that movement of the pendulum allows an optical sensing means to produce digital signals which can be counted and displayed by appropriate circuitry. This product provides one-half degree resolution over a range of plus or minus one hundred twenty degrees from level. Two other techniques for measuring surface angles are described in U.S. Pat. No. 4,077,132 issued to Erickson and U.S. Pat. No. 4,094,073 issued to Parra. There are two main shortcomings observed in the digital inclinometer systems mentioned above. First, the conversion from an analog to digital signal (with the exception of the Heath unit) has associated with it the problems of thermal drift and linearity. Second, there is a limited angular rotation over which an accurate and reliable readout can be obtained.
It should be indicated that using an optical means to detect the position of a bubble in a spirit level has been described in several patents, but the inventor is not aware of its use in the manner of the present invention as described below. Several U.S. patents which describe this technique are: Sweet U.S. Pat. No. 3,371,424, Gooley, U.S. Pat. No. 3,863,067, Beer, U.S. Pat. No. 4,110,609, and Kramer, U.S. Pat. No. 4,154,000.